The field of low temperature processes i.e., those at temperatures below about -100.degree. F. used for the recovery of liquefied gases or for the separation of gases such as air has grown to considerable importance in recent years. The products have found considerable usage in a variety of operations. For example, oxygen is recovered from liquefied air and is used in basic oxygen furnaces and open hearth furnaces found in steel mills and it is used to enrich the air for fuels suited for the field of welding and cutting of metals. Nitrogen is recovered from air and is used as an inert gas, as a refrigerant or as a raw material for the synthesis of ammonia. Argon and other rare gases are recovered from air and used for welding purposes and display signs.
Recently, in view of the domestic energy shortage, and high demands at certain times, storage vessels and cryogenic cycles have been developed for providing both base and peak load quantities of liquefied natural gas during these periods. Consequently, large quantities of natural gas are liquified near the well site and shipped in liquid form to points of usage.
In these low temperature processes, large quantities of refrigeration are required in order to liquefy the gases. Typically, three processes have been used for effecting refrigeration of gases and comprise: (1) heat exchange of gas to be refrigerated against the vaporization of a liquid, (2) the utilization of the Joule-Thompson effect in gases, and (3) the expansion of gas in an engine doing external work. A widely used method for large scale production of gas is the third method in which external work is removed on expansion of the gas in expansion engine. This approach is often referred to as an isentropic expansion whereas the Joule-Thompson method is referred to as an isenthalpic expansion.